Coatings are often applied to substrates to protect them against environmental damage and protect their lifetime, e.g., for scratch resistance, abrasion resistance and solvent/water resistance, or to provide a decorative finish.
Historically, protective materials comprising thermoplastic and thermosetting polymers were applied from solvents which were subsequently removed by evaporation. However, changing safety, health and environment legislation have restricted solvent emissions. Solvent recycling processes have been used but these processes are expensive to install and operate, and are not 100% effective. Therefore, solvent-free coating materials and lacquers have become known. Particularly favourable are coating materials and lacquers which are UV curable because crosslinking is very fast compared to other methods, such as moisture curing or thermal curing. Typical UV lacquers or varnishes are applied at room temperature and consist of epoxy acrylate, polyester acrylate or urethane acrylate oligomers combined with acrylate functional monomers in the presence of a photoinitiator. Under suitable UV wavelengths, the photoinitiators produce free-radicals which polymerise the acrylate functional groups to produce a crosslinked network. Relatively high concentrations of low viscosity reactive monomers have to be used to reduce the viscosity of the coating composition in order to get good flow and levelling at room temperature. In addition, these low viscosity, low molecular weight monomers, e.g., hexanediol diacrylate (HDDA), tripropylene glycol diacrylate (TPGDA) and hydroxyl ethyl methacrylate (HEMA), can penetrate into porous substrates such as wood and then become too deep to be cured by UV radiation. Uncured monomer in the pores of substrates can give safety, health and environmental problems, e.g., when the materials are cut or sanded.
In order to protect substrates in wood-working and furniture applications, UV-curable varnishes are often used and applied by roller, or sometimes by spraying. However, quite often three to four layers of varnish are applied. Typically, only layers of 10 to 20 μm thick can be applied due to the low viscosity and each layer requires sanding before application of the next. The process is time consuming, expensive and requires lots of space.
The use of a solvent-free reactive hot melt layer based on polyurethane and hardened by atmospheric humidity has been suggested. While this method is advantageous in that desired layer thicknesses can be applied in a single operation, curing takes at least several days to occur because it depends on moisture and the coated part cannot be processed or packaged rapidly.
This problem may be reduced by either applying a layer of hot melt that can be cured through both radiation and moisture or by applying a UV curable lacquer on top of the moisture curable hot melt. In the former method, the polyurethane hot melt partly cures via UV radiation enabling handling/processing and then the complete curing of the melt mass takes place via moisture. In the latter method, the surface UV lacquer is immediately dry but the bulk of the hot melt coating still requires at least several days to cure via moisture. As with the use of any moisture curable polyurethane hot melt, use of a dual UV/moisture cure system requires use of a material that must be packaged in a special way—i.e., in the absence of moisture (e.g., under vacuum in expensive metal/plastic packaging that is a moisture barrier). Another problem is that when the material is heated and applied from a roller coater, thermal stability is limited. This results from moisture from the atmosphere penetrating the adhesive and reacting with the isocyanate groups. This leads to an increase in molecular weight while on the roller and application problems, e.g., stringing or filament formation that can produce fouling of substrates/application equipment.
There continues to be a need in the art for alternative types of coating compositions. The current invention addresses this need.